1. Field of the Invention:
The present invention relates to improvements in an exhaust boiler in which steam is generated by making use of an exhaust gas of a gas turbine using natural gas or heavy oil as fuel as a heat source, and which is of the type that a denitrification apparatus is assembled therein.
2. Description of the Prior Art
In order to reduce NO.sub.x (nitrogen oxides) in an exhaust gas of a gas turbine, frequently a denitrification apparatus was assembled in an exhaust boiler. FIG. 3 is a system diagram showing one example of such exhaust boilers in the prior art, FIG. 5 is a diagram showing temperatures at the respective portions in the exhaust boiler, and in FIG. 3 reference numeral 20 designates an exhaust gas flow passageway, numeral 1 designates a superheater, numeral 2 designates a high-pressure steam generator, numeral 3 designates a denitrification apparatus, numeral 4 designates a high-pressure economizer, numeral 5 designates a low-pressure steam generator, numeral 6 designates a low-pressure economizer, numeral 7 designates an ammonia injection system and numeral 8 designates a stack.
However, as a result of the assembly of the denitrification apparatus 3, unreacted ammonia always would be generated in the section of the denitrification apparatus. Consequently, in the case where a sulfur content is contained in the fuel of the gas turbine, heat absorption is allowed only up to the temperature region where acidic ammonium sulfate produced from SO.sub.2 in the combustion gas and the unreacted ammonia can exist stably in a solid phase. (It is said that acidic ammonium sulfate is present in a liquid phase at a temperature of 150.degree. C. or lower when a molecular ratio is NH.sub.3 /H.sub.2 SO.sub.4 .ltoreq.1.1. If this acidic sulfate is present in a liquid phase within an exhaust boiler tube, this would serve as a binder and dust or the like in the exhaust gas would secure to the heat transfer tube, resulting not only in deterioration of the heat transfer effect of the tube but also draft loss of the exhaust boiler, and sometimes reduction of the output of the gas turbine would result. In addition, there is a problem of corrosion of the heat transfer tube caused by ammonium sulfate in the liquid phase.)
Accordingly, in the prior art, in an exhaust boiler for a gas turbine in which fuel not containing a sulfur content and fuel containing a sulfur content are burnt either individually or in mixture, in, view of the above countermeasure for avoiding acidic ammonium sulfate in the liquid phase, only an exhaust boiler having a heat transfer surface arrangement such that the exhaust gas is discharged at such a high gas temperature that acidic ammonium sulfate is present in a solid phase (a temperature above the dashed line in FIG. 5) could be contemplated. More particularly, while the heat transfer surface arrangement as shown in FIG. 3 was allowed in the case where the problem of acidic ammonium sulfate was not present, in the case where the problem of acidic ammonium sulfate was present, one was compelled to employ the heat transfer surface arrangement as shown in FIG. 4. In FIG. 4, reference numeral 31 designates a high-pressure steam drum, numeral 32 designates a high-pressure saturated steam tube, numeral 33 designates a circulation pump, numeral 34 designates a mixer, and numeral 35 designates a condensed water line.
In order to raise the temperature at the highpressure economizer 4, condensed water and water from the high-pressure steam drum 31 are mixed in this mixer 34. As another method for raising the inlet temperature of the high-pressure economizer 4, a method of heating by steam is known. In that case, in place of the system of the circulation pump 33 in FIG. 4, a steam turbine extraction system or a high-pressure main steam system would be led to the mixer 34.
In the case where fuel containing a sulfur content and fuel not containing a sulfur content were respectively and individually burnt in the same gas turbine, in the prior art a heat transfer surface arrangement of an exhaust boiler was determined in view of a countermeasure for acidic ammonium sulfate. Accordingly, there was an inconvenience that even in the event that fuel not containing a sulfur content was employed, sufficient heat recovery could not be achieved because the heat transfer surfaces were fixed.